What it shows
Steel wool rusts by using up the oxygen in a closed jar, so water rises in the jar to take the place of the oxygen. The water rises by about one fifth of the air column, showing that air is about 21 percent oxygen.
The science
Iron reacts slowly with oxygen and water to form hydrated iron oxide, which we call rust. In a closed container the iron uses up only the oxygen, since nitrogen and the other gases do not react. The pressure inside the jar falls and the outside air pressure pushes water in until the lost volume is replaced. Dry air is about 78 percent nitrogen, 21 percent oxygen and 1 percent other gases, so the water should rise by about one fifth. Vinegar removes the protective oil and speeds the rusting.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A tall clear glass or bottle with a straight side, cut-off 1 litre bottle is fine
- A bowl or tray of water
- A pad of plain steel wool, about 5 g
- White vinegar, about 100 ml
- A ruler
- A marker pen
- A small plate or tape
Safety
For this project: An adult must cut the bottle and handle the steel wool, which has sharp fibres: wear gloves. Rinse and dry hands afterwards. Do not use soapy steel wool pads.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Dip the steel wool in vinegar for one minute to remove the oil coat, and squeeze out the extra liquid.
Push the damp wool to the bottom of the glass so that it stays there, and tape or pack it in place.
Measure the height of the glass inside and write it down. A straight-sided glass makes the measurement simple.
Turn the glass upside down in the bowl of water so that the mouth is under water and the glass stands straight.
Mark the water level inside the glass with the marker, and keep it for 4 days in a place away from direct sun.
Each day mark the level of water in the glass and measure its height above the water in the bowl.
Once the level stops rising, measure the height of the air column and the height risen, and calculate: rise divided by original air height times 100.
Repeat with a second glass and compare. A common mistake is leaving air trapped between the wool and the glass bottom, which gives errors.
What you should see
The steel wool turns brown with rust and water rises by about 18 to 21 percent of the air height, close to the 21 percent oxygen in air.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the water rise in the glass?
Oxygen is used up by the rusting, so the pressure falls and outside air pressure pushes the water in.
2Why does the water not fill the whole glass?
Only oxygen is used up; nitrogen and the other gases remain.
3Why do we dip the wool in vinegar?
Vinegar removes the oily layer so rusting begins quickly.
4What is rust chemically?
Hydrated iron(III) oxide.
Take it further
Repeat the test with a sealed jar that has a drop of salt water on the wool and see whether it rusts faster.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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